A speaker system reproducing sound by adding vibration to an acoustic diaphragm by using a magnetostrictive actuator has been heretofore proposed (refer to JP-A-2007-166027 (Patent Document 1)).
As shown in FIG. 1, a cylindrical pipe 2 made of acrylic resin and so on is supported vertically on a disk-shaped base casing 3 and actuators 4 are arranged at four places of the base casing 3 at equal intervals in a speaker system 1.
In the speaker system 1, driving rods 4A of respective actuators 4 are allowed to abut on a lower end face of the pipe 2 and the actuators 4 are driven by an audio signal to add vibration to a vertical direction to the lower end face of the pipe 2.
At this time, the lower end face of the pipe 2 is excited by longitudinal waves. Elastic waves (vibration) is propagated in a surface direction (direction parallel to the surface) of the pipe 2 to be mixed waves in which longitudinal waves and transverse waves are mixed, and interaction of Poisson's ratio of the pipe 2 representing the relation between strain in an expansion and contraction direction of the elastic waves and strain orthogonal to the expansion and contraction direction can be obtained. As a result, vibration in an in-plane direction (direction vertical to the surface) is excited with uniform magnitude over the whole surface of the pipe 2 to emit sound waves, thereby forming a sound image with a uniform sense of extent over the whole pipe 2 in the height direction.
Patent Document 1 also shows that a normal speaker unit 6 is attached to an opening 5 at the center of the base casing 3 though omitted in the above speaker system 1.
In this case, the pipe 2 is configured to function as a tweeter taking charge of high-ranges of an audio frequency band and the normal speaker unit 6 is configured to function as a woofer taking charge of low-ranges of the audio frequency band.